motive: cohesive powders look like asteroid surfaces and ... · the microgravity surfaces of small...

Motive: Cohesive Powders Look Like Asteroid Surfaces and Can Refine Asteroid Simulations G. Devaud3, A. Fischer3, D. D. Durda1, P. Sánchez2, D. J. Scheeres2, P. F. Kaptchen3, S. E. Roark3, and R. Dissly3. 1Southwest Research Institute, 1050 Walnut Street, Suite 300 Boulder, CO 80302

(durda@boulder.swri.edu), 2University of Colorado, Boulder CO 80309, 3Ball Aerospace and Technology Corporation, Boulder CO 80301 Corresponding Author Jenny Devaud: gdevaud@ball.com

Jenny Devaud

High-resolution images of the near-Earth asteroids 433 Eros and 25143 Itokawa, returned by the

NASA NEAR-Shoemaker and JAXA Hayabusa missions, respectively, and spacecraft-

reconnaissance-quality radar observations of several other near-Earth asteroids (NEAs), have

revealed surfaces covered in coarse regoliths. The largest size fractions in these regoliths are

boulders ranging in size up to many 10s of meters across. While certainly some or even many of

these boulders must be coherent, solid blocks of competent rock (solid rocky meteorites derived

from NEAs and their main-belt precursors fall to Earth on a regular basis), the possibility remains

that some others might in fact be composed of loosely-bonded clods of smaller regolith particles

held together by self-cohesive forces that can play a dominant role in affecting regolith properties

and processes in the microgravity environments on these small objects.

Scheeres et al. [1] performed a survey of the known relevant forces that act on grains and particles

on asteroid surfaces (including gravitational and rotational accelerations, Coulomb friction, self

gravitation, electrostatics, solar radiation pressure forces, and surface contact cohesive forces),

developed their analytical form and relevant constants for the space environment, and considered

how these forces scale relative to each other. Among key findings of that study is the result that

van der Waals cohesive forces should be a significant effect for the mechanics and evolution of

asteroid surfaces and interiors and that asteroid regolith may be better described by cohesive

powders (for a familiar analogy, consider the mechanical properties of bread flour) than by

traditional analyses assuming cohesionless grains.

This observation implies that regoliths composed of impact debris of those sizes should behave on

the microgravity surfaces of small asteroids like flour or other cohesive powders do in the 1-g

environment here on Earth. To this end, we have been conducting a series of laboratory

experiments to investigate the role of cohesion in governing regolith processes and

geomorphological expression on small solar system bodies. Our goal is to develop an improved

understanding of the geomorphological expression of granular media in the microgravity

environments of regoliths on small asteroids, and to quantify the range of expected mechanical

properties of such regoliths.

Based on a rigorous comparison of forces it can be shown that van der Waals cohesive forces

between millimeter to centimeter-sized grains on asteroids ranging in size from Eros to Itokawa,

respectively, may exceed their ambient weight several-fold.

This observation implies that regoliths composed of impact debris of those sizes should behave

on the microgravity surfaces of small asteroids like flour or other cohesive powders do in the 1-g

environment here on Earth.

10 micron scale on Earth

mm-cm scale on Itokawa

Pile Tip

Pile Two Wall Compression and Tip

Four Wall Vibration and Tip

One Moving Wall Progression

Glass Microspheres

White Flour

Sand

JSC-1AF

Toner

Reproduction of Published Results

Published Results

Goal: Create High Accuracy Asteroid Simulations Summary:10 μm diameter Glass Microspheres in 1g can be used to approximate the mm-cm dust particles in μ g (which comprise asteroids). Using data from Glass Microsphere lab experiments in vacuum, asteroid simulations can be refined and calibrated.

10 μm scale dust in 1g can be used to approximate mm-cm scale dust in μg

Because powder simulations use spherical models to approximate dust particles, glass microspheres are the most appropriate real world cohesive powder to study.

Dust Particles in Simulation Frequently Studied Dust Particles (SEM x5000 Mag)

Using various approaches, we can recreate asteroid topography in cohesive powders under ½ Torr Vacuum Various Asteroid features can be found in test results shown outlined in red

We can calibrate simulation of mm-cm scale dust in μg using real world 10 μm scale Dust Piles in 1g from lab experiments

Spinning Pile

Sieve Dust into Pile Pump Down To Vacuum Tip Pile

Sieve Dust into Pile Pump Down To Vacuum and Compress Pile

Tip Pile

Sieve Dust into 4 Wall Box Pump Down To Vacuum and Vibrate

Remove 2 Walls and Tip Pile

Sieve Dust into 4 Wall Box

Sieve Dust into Pile Pump Down To Vacuum Tip Pile

Move 1 Wall and Observe Column Collapse

Asteroid

Experimental Rubble Pile Rubble Pile “Boulders” “Boulders” Isolated

“Boulders” Graphed

Conclusions: 10 μm diameter Glass Microspheres in 1g can be used to approximate the mm-cm dust particles in μ g (which comprise asteroids). Using data from Glass Microsphere lab experiments in vacuum, asteroid simulations can be refined and calibrated.